A lithium-sulfur secondary cell using a sulfur substance as a cathode active material and lithium (Li) as an anode active material has been developed. Theoretical capacity densities of lithium and sulfur are about 3862 mAh/g and about 1672 mAh/g, respectively. It is supposed to provide a secondary cell having a very high energy density. However, there are the reasons why the lithium-sulfur secondary cell is not practically used at present:    (1) A utilization ratio of sulfur as the cathode active material is low.    (2) A charge-discharge cycle property is low.It cannot fully utilize a significantly great theoretical capacity density characterized in the lithium-sulfur secondary cell.
One of the reasons for (1) may be as follows: Upon discharge, lithium ions are reacted with S8 sulfur in the cathode to generate sulfide Li2Sx. If the reaction proceeds, the value of x is changed from 8 to 4, 2, or 1. Here, if the value of x is 8, 4, or 2, sulfide Li2Sx is partly dissolved in an electrolyte. Then, if the reaction proceeds, and the sulfide dissolved becomes Li2S (i.e., x=1), the sulfide is not dissolved in the electrolyte, and precipitated to damage the electrode. Therefore, at present, only the sulfide having the value x of up to about 2 (theoretical capacity density: 836 mAh/g) can be used for discharge.
In relation to (2), the reason may be that sulfur (for example, S8 sulfur) is an insulator having electrical resistivity of 10−30 Ω·cm−1, and that polysulfide is eluted in electrolyte. Furthermore, the polysulfide eluted in the electrolyte is reduced at an anode during charging, and polysulfide having shorter sulfur chains are generated, migrates to a cathode and is oxidized again, which is called as a redox shuttle reaction. Due to the redox shuttle reaction, the cut-off voltage is not reached, and an overcharge state is induced.
Patent Document 1: Japanese Patent Application Laid-open No. 2010-257689
Non-Patent Document 1: L. Nazar et.al., Nature Materials, 8, 500, 2009
Non-Patent Document 2: M. Watanabe et.al., Chem.Commun., 47, 8157-8159 (2011)